This invention relates to a high speed turning machine for machining a rotating workpiece to have a predetermined profile surface by means of a cutting tool that is reciprocated rapidly toward and from the workpiece surface.
The present invention is directed to high speed turning tools in which the workpiece is rotated at high speed and a cutting tool is reciprocated at high speed into and from engagement with the rotating workpiece surface to provide a particular profile thereto. The tool is brought into engagement with a turning surface on the workpiece and the tool movement is controlled and programmed to form a non-circular surface such as for a piston skirt, a cam shaft, or journals etc. for automotive engines. As will be explained hereinafter, the tool may be brought into contact with an end face of the rotating workpiece to perform a facing operation or into contact with an internal surface in a bore to a boring operation on the rotating workpiece. In the first instance, of a piston turning operation, the tool moves transverse to the spinning axis of the workpiece, whereas in the end-facing instance, the tool moves parallel to the spinning axis of the workpiece.
The present invention will be described hereafter principally in connection with a piston turning operation in which the workpiece is rotated by a spindle assembly with the tool moving at right angles to the turning axis to machine the workpiece and the toolholder is carried axially in a direction parallel to the turning axis to cut a non-circular profile along the outer surface of the piston skirt. In prior machines of this general kind, the tool was mounted on the end of a steel shaft which wa mounted for sliding movement within a linear motor with the armature also being carried on the steel shaft. The linear motor was a high response linear motor, the stroke of which was controlled by a computer program operating to control the tool tip location relative to a point on the rotating part. Variations from the profile geometry are made automatically by modifying the computer program. Displacement errors are gauged and out of tolerance conditions are corrected automatically through a close looped feedback system. The particular system was found to be fairly responsive to the needs of the customer. These machines though, in actual use, suffered from a number of shortcomings, including a lack of stiffness, a relatively high moment of inertia, a limited response speed and relatively high friction.
The response of the toolholder and its servo become a limiting factor in these prior art machines because of their bandwidth. The bandwidth of the toolholder and servo system is defined as the range of frequencies with which the assembly will respond when forced with a control signal. The bandwidth of a toolholder and servo depend upon five basic factors which are the maximum cutting force required, the inertia of the toolholder, the toolholder range of motion, frictional forces and the servo force capacity.
Generally, the higher the inertia of the toolholder and servo, the lower will be the inherent bandwidth of the combination. For a particular physical configuration, the maximum frequency of response will be limited by this factor. Reducing inertia (particularly mass) of the moving parts of a toolholder and servo combination will increase this limit for a servo with a specific force capacity. The present invention is directed to providing a very substantial increase in stiffness, a reduction in moment of inertia, an increase in speed and in accuracy of cutting to provide a higher production and better finish on the workpiece, as will be described hereinafter.
In prior art turning machines of this type, the tool was carried on the end of a metal shaft which reciprocated on bearings along a linear path and which carried an armature of a linear motor. The high moment of inertia of the metal shaft carrying the tool and the linear motor armature caused the tool to overshoot in the direction of its movement. Also, to provide greater maximum cutting forces, the tool and toolholder were relatively massive resulting in this greater moment of inertia thereby reducing the response time to stop the tool, to reverse its direction of travel without a large overshoot travel, and to accelerate the tool's travel in the opposite direction. When excessive electrical power was applied to the tool and armature shaft to attempt to get greater cutting forces in these prior cutting machines, the tool shaft tended to overshoot and then vibrate. Thus, there is a need to provide a new and improved toolholder and linear motor arrangement which very substantially reduces the moment of inertia of the toolholder and tool.
One conventional piston turning machine with the reciprocating tool shaft carrying the armature of the linear move had a limit stiffness of about 150,000 lbs in. Stiffness as used herein defined as the resistance to tool deflection under cutting load. The greater stiffness not only results in more cut material being removed but also results in better control over where the tool point is located at any given point in time. For reasons to be explained hereinafter, the stiffness of the tool of this invention over the prior art machine may be almost twenty-fold. This increased stiffness and results in better surface finishes and can eliminate one or more finish cuts required with the conventional equipment thereby substantially increasing the rate of production.
Usually, bandwidth is lost when it is desired to increase the cutting force because more massive and powerful servos are used to generate the cutting force but at the expense of increasing the inertia of the system. Further, the more heavily loaded a servo mechanism becomes, the more sluggish is its response to a demand for accelerations and direction changes. As will be explained in connection with this invention, the cutting force may be increased substantially by mounting the armature and toolholder relative to the tool to provide a mechanical advantage in excess of two. Thus, the cutting force may be increased over the prior art systems without the use of more massive motors or servos.
These conventional piston turning machines had a heavy metal shaft carrying the cutting tool and the armature; and the shaft was mounted in bearings or slide bushings which generate considerable friction. Friction is a loss that requires electrical energy and force from a motor to be overcome. Frictional forces reduce system bandwidths because of the thermal and mechanical energy losses they produce. Decreasing frictional forces will produce a higher bandwidth. An earlier prior art machine included a camming wedge transfer means that produce high friction and wear on the wedge surfaces. High wear surfaces add looseness into the system that must be accounted for. The present invention mounts the toolholder and the linear motor armature in a manner to reduce the friction very substantially and thereby maintain a high bandwidth.
The response or range of frequency or bandwidth determines how rapidly the tool can be moved and thereby is an important factor in the rate of machining. If the tool and tool holder can not be moved rapidly and through small increments of movement in these machines, the rotational speed of the workpiece must be reduced. The conventional machine being described herein reciprocated the tool about 40 cps and the workpiece was rotated at 1,300 rpm. The other prior art machine, which is described herein, had a tool reciprocate within the range of 60 to 70 cps with the work piece rotating at about 2,200 rpm. As will be explained in greater detail hereinafter, the present invention provides a substantial increase in both tool reciprocation, e.g. 100-130 cps and a workpiece rotation in excess of 3,000 rpm. These higher speeds for cutting result in finer finishes, more machining per unit time and better production of finished workpieces.
As a general principle, the longer the stroke or range of motion required by the toolholder and servo, the lower the bandwidth. The range of motion of the tool of the present invention may be kept quite small, e.g. about one-half inch or less. Heretofore, prior art machines described herein had a longer stroke or range of motion, e.g. about one inch that lowered the bandwidth.
From the foregoing, it will be seen that the present invention addresses and provides improvements in the five basic factors for the bandwidth of a toolholder and servo, which are: the maximum cutting force required, the inertia of the toolholder, toolholder range of motion, frictional forces and the servo capacity.
Thus, a general object of the invention is to provide a new and improved high speed turning machine of the foregoing kind.